Rhodopsin Mutations Research Articles

Gene therapy to rescue retinal degeneration caused by mutations in rhodopsin.

Retinal gene therapy has proven safe and at least partially successful in clinical trials and in numerous animal models. Gene therapy requires characterization of the progression of the disease and understanding of its genetic cause. Testing gene therapies usually requires an animal model that recapitulates the key features of the human disease, though photoreceptors and cells of the retinal pigment epithelium produced from patient-derived stem cells may provide an alternative test system for retinal gene therapy. Gene therapy also requires a delivery system that introduces the therapeutic gene to the correct cell type and does not cause unintended damage to the tissue. Current systems being tested in the eye are nanoparticles, pseudotyped lentiviruses, and adeno-associated virus (AAV) of various serotypes. Here, we describe the techniques of AAV vector design as well as the in vivo and ex vivo tests necessary for assessing the efficacy of retinal gene therapy to treat retinal degeneration caused by mutations in the rhodopsin gene.

Functional Characterization of Human Rhodopsin Mutations by Fluorescence Imaging

Rhodopsin is the membrane receptor responsible for photoreception in the vertebrate retina. Over 120 point mutations in rhodopsin are found to be related with autosomal dominant retinitis pigmentosa (ADRP) and the congenital stationary night blindness (CNSB). Despite of several mutations with intense studies, like P23H, a majority of rhodopsin mutations still need further investigations. In order to have extensive and quick functional characterization of these mutations, here we utilize fluorescence imaging to monitor rhodopsin cellular distribution, which reveals to us much useful information, like if rhodopsin has normal transportation to the cell membrane, interrrupted glycosylation or protein aggregates formation. The experiments are carried out through the following process: First, a series of human rhodopsin mutations were constructed, which include mutations responsible for both ADRP and CNSB, like G89D and G90D. Second, wild-type rhodopsin was expressed in 293S GnTi- cells with homogeneous N-glycosylation for protein detection, T-REx293 cells for glycosylation analysis, and Hela cells for immunofluorescence imaging seperately. Third, we also engineered a fusion protein rho-EGFP for in vivo study, with a green fluorescent protein insterted into human rhodopsin. We found that mutations that cause ADRP are usually misfolded and retained in endoplasmic reticulum and thus have low efficiency of 11-cis-retinal binding. And G90D mutant (causing CNSB) that was considered to have correct protein folding, however, showed an astonishing tendency to form inclusion bodies in our study, which may be related with its interrupted glycosylation in Golgi apparatus. In a word, we have established an effective and convenient system to investigate rhodopsin synthesis and transportation both in vitro and in vivo.

Gene therapy to treat inherited and complex retinal degenerative diseases

Gene therapy to treat inherited and complex retinal degenerative diseases

High-throughput screening assays to identify small molecules preventing photoreceptor degeneration caused by the rhodopsin P23H mutation.

High-throughput screening (HTS) is one of the major techniques for discovering promising molecules for drug development. Rhodopsin mutations cause the most common autosomal dominant form of retinitis pigmentosa, an inherited retinal degenerative disease that currently has no effective treatment. To find an optimal pharmacological treatment for rhodopsin-associated retinitis pigmentosa, we performed two cell-based HTSs with mammalian cells expressing the P23H rod opsin mutant and identified two sets of novel compounds for further validation and characterization. The first HTS screen identified pharmacological chaperones of P23H opsin that increased its translocation from the endoplasmic reticulum to the plasma membrane. The second HTS screen selected small molecules that enhanced the clearance of the mutant opsin while vision could be sustained by the healthy gene allele expressing wild-type rhodopsin. Here we describe the methodology of these two HTS assays in detail.

Rapid and facile recombinant expression of bovine rhodopsin in HEK293S GnTI(-) cells using a PiggyBac inducible system.

Rhodopsin is a class A G protein-coupled receptor (GPCR) that provides important insights into the structure and function of the GPCR superfamily. Bovine rhodopsin is widely used as a model for GPCRs and was the first GPCR whose X-ray crystal structure was solved. One of the advantages of rhodopsin is that it is abundant in native tissue, and as a result, milligram quantities can be purified from the retinal rod cells of bovine eyes. Nonetheless, the study of GPCR conformation and dynamics, e.g., by electron paramagnetic resonance or (19)F nuclear magnetic resonance spectroscopy, typically requires mutagenesis to enable site-directed labeling of the protein. Mutations are also of great importance as they can stabilize the receptor and can be necessary to study different receptor conformations. Recombinant production of rhodopsins for biophysical studies has been achieved in different systems, including mammalian, insect, and yeast cells in culture, and from Drosophila melanogaster and Caenorhabditis elegans tissue. The piggyBac (PB) transposon system is used for gene delivery into a variety of cell types (e.g., HEK293 and CHO cells, fibroblasts, stem cells) and living organisms (e.g., honeybees, pigs, chicken, mice). Recently, the PB transposon has been described as an efficient tool for inducible protein expression in HEK293T and HEK293S N-acetylglucosaminyltransferase I-deficient (GnTI(-)) cells. This chapter describes a protocol for using the PB-based system for inducible expression of bovine rhodopsin in HEK293S GnTI(-) cells. Using this protocol, we expressed and purified 26 rhodopsin mutants to be used for site-directed spin labeling.

Mammalian expression, purification, and crystallization of rhodopsin variants.

After 25 years of intensive research, the understanding of how photoreceptors in the eye perceive light and convert it into nerve signals has largely advanced. Central to this is the structural and mechanistic exploration of the G protein-coupled receptor rhodopsin acting as a dim-light sensing pigment in the retina. Investigation of rhodopsin by X-ray crystallographic, electron microscopic, and biochemical means depends on the ability to produce and isolate pure rhodopsin protein. Robust and well-defined protocols permit the production and crystallization of rhodopsin variants to investigate the inactive ground, the fully activated metarhodopsin II state, or disease-causing rhodopsin mutations. This chapter details how we express and purify biologically active variants of rhodopsin from HEK293S GnTI(-) cells in a quality and quantity suitable for biochemical assays, crystallization, and structure determination.

Correlation between SD-OCT, immunocytochemistry and functional findings in an animal model of retinal degeneration

Purpose: The P23H rhodopsin mutation is an autosomal dominant cause of retinitis pigmentosa (RP). The degeneration can be tracked using different anatomical and functional methods. In our case, we evaluated the anatomical changes using Spectral-Domain Optical Coherence Tomography (SD-OCT) and correlated the findings with retinal thickness values determined by immunocytochemistry.Methods: Pigmented rats heterozygous for the P23H mutation, with ages between P18 and P180 were studied. Function was assessed by means of optomotor testing and ERGs. Retinal thicknesses measurements, autofluorescence and fluorescein angiography were performed using Spectralis OCT. Retinas were studied by means of immunohistochemistry. Results: Between P30 and P180, visual acuity decreased from 0.500 to 0.182 cycles per degree (cyc/deg) and contrast sensitivity decreased from 54.56 to 2.98 for a spatial frequency of 0.089 cyc/deg. Only cone-driven b-wave responses reached developmental maturity. Flicker fusions were also comparable at P29 (42 Hz). Double flash-isolated rod-driven responses were already affected at P29. Photopic responses revealed deterioration after P29.A reduction in retinal thicknesses and morphological modifications were seen in OCT sections. Statistically significant differences were found in all evaluated thicknesses. Autofluorescence was seen in P23H rats as sparse dots. Immunocytochemistry showed a progressive decrease in the outer nuclear layer (ONL), and morphological changes. Although anatomical thickness measures were significantly lower than OCT values, there was a very strong correlation between the values measured by both techniques.Conclusions: In pigmented P23H rats, a progressive deterioration occurs in both retinal function and anatomy. Anatomical changes can be effectively evaluated using SD-OCT and immunocytochemistry, with a good correlation between their values, thus making SD-OCT an important tool for research in retinal degeneration.

An activated unfolded protein response promotes retinal degeneration and triggers an inflammatory response in the mouse retina.

Recent studies on the endoplasmic reticulum stress have shown that the unfolded protein response (UPR) is involved in the pathogenesis of inherited retinal degeneration caused by mutant rhodopsin. However, the main question of whether UPR activation actually triggers retinal degeneration remains to be addressed. Thus, in this study, we created a mouse model for retinal degeneration caused by a persistently activated UPR to assess the physiological and morphological parameters associated with this disease state and to highlight a potential mechanism by which the UPR can promote retinal degeneration. We performed an intraocular injection in C57BL6 mice with a known unfolded protein response (UPR) inducer, tunicamycin (Tn) and examined animals by electroretinography (ERG), spectral domain optical coherence tomography (SD-OCT) and histological analyses. We detected a significant loss of photoreceptor function (over 60%) and retinal structure (35%) 30 days post treatment. Analysis of retinal protein extracts demonstrated a significant upregulation of inflammatory markers including interleukin-1β (IL-1β), IL-6, tumor necrosis factor-α (TNF-α), monocyte chemoattractant protein-1 (MCP-1) and IBA1. Similarly, we detected a strong inflammatory response in mice expressing either Ter349Glu or T17M rhodopsin (RHO). These mutant rhodopsin species induce severe retinal degeneration and T17M rhodopsin elicits UPR activation when expressed in mice. RNA and protein analysis revealed a significant upregulation of pro- and anti-inflammatory markers such as IL-1β, IL-6, p65 nuclear factor kappa B (NF-kB) and MCP-1, as well as activation of F4/80 and IBA1 microglial markers in both the retinas expressing mutant rhodopsins. We then assessed if the Tn-induced inflammatory marker IL-1β was capable of inducing retinal degeneration by injecting C57BL6 mice with a recombinant IL-1β. We observed ~19% reduction in ERG a-wave amplitudes and a 29% loss of photoreceptor cells compared with control retinas, suggesting a potential link between pro-inflammatory cytokines and retinal pathophysiological effects. Our work demonstrates that in the context of an established animal model for ocular disease, the persistent activation of the UPR could be responsible for promoting retinal degeneration via the UPR-induced pro-inflammatory cytokine IL-1β.

Chemical chaperone 4-phenylbutyrate prevents endoplasmic reticulum stress induced by T17M rhodopsin.

BackgroundRhodopsin mutations are associated with the autosomal dominant form of retinitis pigmentosa. T17M mutation in rhodopsin predisposes cells to endoplasmic reticulum (ER) stress and induces cell death. This study aimed to examine whether chemical chaperone 4-phenylbutyrate prevents ER stress induced by rhodopsin T17M.ResultsARPE-19 cells were transfected with myc-tagged wild-type (WT) and T17M rhodopsin constructs. Turnover of WT and T17M rhodopsin was measured by cycloheximide chase analysis. The activity of ubiquitin-proteasome system was evaluated by GFPU reporter. We found that T17M rhodopsin was misfolded, ubiqutinated and eliminated by ER-associated degradation pathway (ERAD) in ARPE-19 cells. Accumulated T17M rhodopsin induced unfolded protein response, but had no effect on the activity of ubiquitin proteasome system. Moreover, chemical chaperone 4-phenylbutyrate facilitated the turnover of T17M rhodopsin and prevented apoptosis and ER stress induced by T17M rhodopsin.ConclusionsChemical chaperone could attenuate UPR signaling and ER stress induced by T17M rhodopsin and has potential therapeutic significance for retinitis pigmentosa.

Differential Light-induced Responses in Sectorial Inherited Retinal Degeneration

Retinitis pigmentosa (RP) is a group of genetically and clinically heterogeneous inherited degenerative retinopathies caused by abnormalities of photoreceptors or retinal pigment epithelium in the retina leading to progressive sight loss. Rhodopsin is the prototypical G-protein-coupled receptor located in the vertebrate retina and is responsible for dim light vision. Here, novel M39R and N55K variants were identified as causing an intriguing sector phenotype of RP in affected patients, with selective degeneration in the inferior retina. To gain insights into the molecular aspects associated with this sector RP phenotype, whose molecular mechanism remains elusive, the mutations were constructed by site-directed mutagenesis, expressed in heterologous systems, and studied by biochemical, spectroscopic, and functional assays. M39R and N55K opsins had variable degrees of chromophore regeneration when compared with WT opsin but showed no gross structural misfolding or altered trafficking. M39R showed a faster rate for transducin activation than WT rhodopsin with a faster metarhodopsinII decay, whereas N55K presented a reduced activation rate and an altered photobleaching pattern. N55K also showed an altered retinal release from the opsin binding pocket upon light exposure, affecting its optimal functional response. Our data suggest that these sector RP mutations cause different protein phenotypes that may be related to their different clinical progression. Overall, these findings illuminate the molecular mechanisms of sector RP associated with rhodopsin mutations.

Prevalence of Rhodopsin mutations in autosomal dominant Retinitis Pigmentosa in Spain: clinical and analytical review in 200 families.

We aimed to determine the prevalence of mutations in the RHO gene in Spanish families with autosomal dominant Retinitis Pigmentosa (adRP), to assess genotype-phenotype correlations and to establish an accurate diagnostic algorithm after 23 years of data collection. Two hundred patients were analysed through a combination of denaturing gradient gel electrophoresis, single-strand conformation polymorphism, genotyping microarray and Sanger sequencing of the RHO gene. Overall, 42 of 200 Spanish adRP families were mutated for RHO (21.0%). Twenty-seven different RHO mutations were detected; seven of them were novel. A genotype-phenotype correlation was established with clinical data from 107 patients. The most prevalent p.Pro347Leu mutation, responsible for 4.5% (9/200) of all mutated adRP families, was associated with a phenotype of early onset and severe course diffuse RP. This retrospective study provides a wide spectrum of mutations in the RHO gene in Spanish patients with adRP. Also, the prevalence of mutations is similar to that reported in European population. Genotyping microarray followed by RHO sequencing is proposed as a first step in molecular diagnosis of adRP Spanish families. An increasing understanding of causal RHO alleles in adRP facilitates disease diagnosis and prognosis, especially for the prevalent p.Pro347Leu mutation.

Robust Endoplasmic Reticulum-Associated Degradation of Rhodopsin Precedes Retinal Degeneration.

Rhodopsin is a G protein-coupled receptor essential for vision and rod photoreceptor viability. Disease-associated rhodopsin mutations, such as P23H rhodopsin, cause rhodopsin protein misfolding and trigger endoplasmic reticulum (ER) stress, activating the unfolded protein response (UPR). The pathophysiologic effects of ER stress and UPR activation on photoreceptors are unclear. Here, by examining P23H rhodopsin knock-in mice, we found that the UPR inositol-requiring enzyme 1 (IRE1) signaling pathway is strongly activated in misfolded rhodopsin-expressing photoreceptors. IRE1 significantly upregulated ER-associated protein degradation (ERAD), triggering pronounced P23H rhodopsin degradation. Rhodopsin protein loss occurred as soon as photoreceptors developed, preceding photoreceptor cell death. By contrast, IRE1 activation did not affect JNK signaling or rhodopsin mRNA levels. Interestingly, pro-apoptotic signaling from the PERK UPR pathway was also not induced. Our findings reveal that an early and significant pathophysiologic effect of ER stress in photoreceptors is the highly efficient elimination of misfolded rhodopsin protein. We propose that early disruption of rhodopsin protein homeostasis in photoreceptors could contribute to retinal degeneration.

Drug-inducible synergistic gene silencing with multiple small hairpin RNA molecules for gene function study in animal model.

Gene targeting is a critical tool for construction of disease models. However, the application of traditional homologous recombination-mediated gene knockout technology is limited by the absence of rapid frequency-guaranteed targeting methods. Although conventional small hairpin RNA (shRNA)-mediated gene silencing offers an alternative for gene targeting, its application is frequently compromised by lower expression efficiency via RNA interference compared to gene knockout. Here we provide an efficient gene targeting strategy involving drug-inducible synergistic silencing with multiple shRNA molecules. On induction, the levels of the target proteins decreased to undetectable levels in all the tested stable transgenic mammalian cell lines, including HEK293 and embryonic stem cell-derived progenies carrying shRNA silencing cassettes. In a transgenic mouse model carrying a silencing cassette targeting the rhodopsin gene, short-time inducer treatment was sufficient to ablate the rhodopsin protein in the retina, resulting in similar retinal phenotypic changes as those observed in rhodopsin mutant mice. Therefore, on a broad basis, this inducible shRNA gene targeting strategy offers a true gene knockout alternative comparable to conventional RNA interference approaches.Electronic supplementary materialThe online version of this article (doi:10.1007/s11248-014-9841-9) contains supplementary material, which is available to authorized users.

Photoactivation-induced instability of rhodopsin mutants T4K and T17M in rod outer segments underlies retinal degeneration in X. laevis transgenic models of retinitis pigmentosa.

Retinitis pigmentosa (RP) is an inherited neurodegenerative disease involving progressive vision loss, and is often linked to mutations in the rhodopsin gene. Mutations that abolish N-terminal glycosylation of rhodopsin (T4K and T17M) cause sector RP in which the inferior retina preferentially degenerates, possibly due to greater light exposure of this region. Transgenic animal models expressing rhodopsin glycosylation mutants also exhibit light exacerbated retinal degeneration (RD). In this study, we used transgenic Xenopus laevis to investigate the pathogenic mechanism connecting light exposure and RD in photoreceptors expressing T4K or T17M rhodopsin. We demonstrate that increasing the thermal stability of these rhodopsins via a novel disulfide bond resulted in significantly less RD. Furthermore, T4K or T17M rhodopsins that were constitutively inactive (due to lack of the chromophore-binding site or dietary deprivation of the chromophore precursor vitamin A) induced less toxicity. In contrast, variants in the active conformation accumulated in the ER and caused RD even in the absence of light. In vitro, T4K and T17M rhodopsins showed reduced ability to regenerate pigment after light exposure. Finally, although multiple amino acid substitutions of T4 abolished glycosylation at N2 but were not toxic, similar substitutions of T17 were not tolerated, suggesting that the carbohydrate moiety at N15 is critical for cell viability. Our results identify a novel pathogenic mechanism in which the glycosylation-deficient rhodopsins are destabilized by light activation. These results have important implications for proposed RP therapies, such as vitamin A supplementation, which may be ineffective or even detrimental for certain RP genotypes.

Inner retinal preservation in rat models of retinal degeneration implanted with subretinal photovoltaic arrays

Photovoltaic arrays (PVA) implanted into the subretinal space of patients with retinitis pigmentosa (RP) are designed to electrically stimulate the remaining inner retinal circuitry in response to incident light, thereby recreating a visual signal when photoreceptor function declines or is lost. Preservation of inner retinal circuitry is critical to the fidelity of this transmitted signal to ganglion cells and beyond to higher visual targets. Post-implantation loss of retinal interneurons or excessive glial scarring could diminish and/or eliminate PVA-evoked signal transmission. As such, assessing the morphology of the inner retina in RP animal models with subretinal PVAs is an important step in defining biocompatibility and predicting success of signal transmission. In this study, we used immunohistochemical methods to qualitatively and quantitatively compare inner retinal morphology after the implantation of a PVA in two RP models: the Royal College of Surgeons (RCS) or transgenic S334ter-line 3 (S334ter-3) rhodopsin mutant rat. Two PVA designs were compared. In the RCS rat, we implanted devices in the subretinal space at 4 weeks of age and histologically examined them at 8 weeks of age and found inner retinal morphology preservation with both PVA devices. In the S334ter-3 rat, we implanted devices at 6–12 weeks of age and again, inner retinal morphology was generally preserved with either PVA design 16–26 weeks post-implantation. Specifically, the length of rod bipolar cells and numbers of cholinergic amacrine cells were maintained along with their characteristic inner plexiform lamination patterns. Throughout the implanted retinas we found nonspecific glial reaction, but none showed additional glial scarring at the implant site. Our results indicate that subretinally implanted PVAs are well-tolerated in rodent RP models and that the inner retinal circuitry is preserved, consistent with our published results showing implant-evoked signal transmission.

Rer1p regulates the ER retention of immature rhodopsin and modulates its intracellular trafficking.

Rhodopsin is a pigment in photoreceptor cells. Some rhodopsin mutations cause the protein to accumulate in the endoplasmic reticulum (ER), leading to photoreceptor degeneration. Although several mutations have been reported, how mutant rhodopsin is retained in the ER remains unclear. In this study, we identified Rer1p as a modulator of ER retention and rhodopsin trafficking. Loss of Rer1p increased the transport of wild-type rhodopsin to post-Golgi compartments. Overexpression of Rer1p caused immature wild-type rhodopsin to accumulate in the ER. Interestingly, the G51R rhodopsin mutant, which has a mutation in the first transmembrane domain and accumulates in the ER, was released to the plasma membrane or lysosomes in Rer1-knockdown cells. Consistent with these results, Rer1p interacted with both wild-type and mutant rhodopsin. These results suggest that Rer1p regulates the ER retention of immature or misfolded rhodopsin and modulates its intracellular trafficking through the early secretory pathway.

The co-chaperone and reductase ERdj5 facilitates rod opsin biogenesis and quality control.

Mutations in rhodopsin, the light-sensitive protein of rod cells, are the most common cause of autosomal dominant retinitis pigmentosa (ADRP). Many rod opsin mutations, such as P23H, lead to misfolding of rod opsin with detrimental effects on photoreceptor function and viability. Misfolded P23H rod opsin and other mutations in the intradiscal domain are characterized by the formation of an incorrect disulphide bond between C185 and C187, as opposed to the correct and highly conserved C110–C187 disulphide bond. Therefore, we tested the hypothesis that incorrect disulphide bond formation might be a factor that affects the biogenesis of rod opsin by studying wild-type (WT) or P23H rod opsin in combination with amino acid substitutions that prevent the formation of incorrect disulphide bonds involving C185. These mutants had altered traffic dynamics, suggesting a requirement for regulation of disulphide bond formation/reduction during rod opsin biogenesis. Here, we show that the BiP co-chaperone and reductase protein ERdj5 (DNAJC10) regulates this process. ERdj5 overexpression promoted the degradation, improved the endoplasmic reticulum mobility and prevented the aggregation of P23H rod opsin. ERdj5 reduction by shRNA delayed rod opsin degradation and promoted aggregation. The reductase and co-chaperone activity of ERdj5 were both required for these effects on P23H rod opsin. Furthermore, mutations in these functional domains acted as dominant negatives that affected WT rod opsin biogenesis. Collectively, these data identify ERdj5 as a member of the proteostasis network that regulates rod opsin biogenesis and supports a role for disulphide bond formation/reduction in rod opsin biogenesis and disease.

Crystallization scale preparation of a stable GPCR signaling complex between constitutively active rhodopsin and G-protein.

The activation of the G-protein transducin (Gt) by rhodopsin (Rho) has been intensively studied for several decades. It is the best understood example of GPCR activation mechanism and serves as a template for other GPCRs. The structure of the Rho/G protein complex, which is transiently formed during the signaling reaction, is of particular interest. It can help understanding the molecular details of how retinal isomerization leads to the G protein activation, as well as shed some light on how GPCR recognizes its cognate G protein. The native Rho/Gt complex isolated from bovine retina suffers from low stability and loss of the retinal ligand. Recently, we reported that constitutively active mutant of rhodopsin E113Q forms a Rho/Gt complex that is stable in detergent solution. Here, we introduce methods for a large scale preparation of the complex formed by the thermo-stabilized and constitutively active rhodopsin mutant N2C/M257Y/D282C(RhoM257Y) and the native Gt purified from bovine retinas. We demonstrate that the light-activated rhodopsin in this complex contains a covalently bound unprotonated retinal and therefore corresponds to the active metarhodopin II state; that the isolated complex is active and dissociates upon addition of GTPγS; and that the stoichiometry corresponds to a 1∶1 molar ratio of rhodopsin to the heterotrimeric G-protein. And finally, we show that the rhodopsin also forms stable complex with Gi. This complex has significantly higher thermostability than RhoM257Y/Gt complex and is resistant to a variety of detergents. Overall, our data suggest that the RhoM257Y/Gi complex is an ideal target for future structural and mechanistic studies of signaling in the visual system.

Mercury-induced dark-state instability and photobleaching alterations of the visual g-protein coupled receptor rhodopsin.

Mercuric compounds were previously shown to affect the visual phototransduction cascade, and this could result in vision impairment. We have analyzed the effect of mercuric chloride on the structure and stability of the dim light vision photoreceptor rhodopsin. For this purpose, we have used both native rhodopsin immunopurified from bovine retinas and a recombinant mutant rhodopsin carrying several Cys to Ser substitutions in order to investigate the potential binding site of mercury on the receptor. Our results show that mercuric chloride dramatically reduces the stability of dark-state rhodopsin and alters the molecular features of the photoactived conformation obtained upon illumination by eliciting the formation of an altered photointermediate. The thermal bleaching kinetics of native and mutant rhodopsin is markedly accelerated by mercury in a concentration-dependent manner, and its chromophore regeneration ability is severely reduced without significantly affecting its G-protein activation capacity. Furthermore, fluorescence spectroscopic measurements on the retinal release process, ensuing illumination, suggest that mercury impairs complete retinal release from the receptor binding pocket. Our results provide further support for the capacity of mercury as a hazardous metal ion with reported deleterious effect on vision and provide a molecular explanation for such an effect at the rhodopsin photoreceptor level. We suggest that mercury could alter vision by acting in a specific manner on the molecular components of the retinoid cycle, particularly by modifying the ability of the visual photoreceptor protein rhodopsin to be regenerated and to be normally photoactivated by light.

The use of induced pluripotent stem cells to reveal pathogenic gene mutations and explore treatments for retinitis pigmentosa.

BackgroundRetinitis pigmentosa (RP) is an inherited human retinal disorder that causes progressive photoreceptor cell loss, leading to severe vision impairment or blindness. However, no effective therapy has been established to date. Although genetic mutations have been identified, the available clinical data are not always sufficient to elucidate the roles of these mutations in disease pathogenesis, a situation that is partially due to differences in genetic backgrounds.ResultsWe generated induced pluripotent stem cells (iPSCs) from an RP patient carrying a rhodopsin mutation (E181K). Using helper-dependent adenoviral vector (HDAdV) gene transfer, the mutation was corrected in the patient’s iPSCs and also introduced into control iPSCs. The cells were then subjected to retinal differentiation; the resulting rod photoreceptor cells were labeled with an Nrl promoter-driven enhanced green fluorescent protein (EGFP)-carrying adenovirus and purified using flow cytometry after 5 weeks of culture. Using this approach, we found a reduced survival rate in the photoreceptor cells with the E181K mutation, which was correlated with the increased expression of endoplasmic reticulum (ER) stress and apoptotic markers. The screening of therapeutic reagents showed that rapamycin, PP242, AICAR, NQDI-1, and salubrinal promoted the survival of the patient’s iPSC-derived photoreceptor cells, with a concomitant reduction in markers of ER stress and apoptosis. Additionally, autophagy markers were found to be correlated with ER stress, suggesting that autophagy was reduced by suppressing ER stress-induced apoptotic changes.ConclusionThe use of RP patient-derived iPSCs combined with genome editing provided a versatile cellular system with which to define the roles of genetic mutations in isogenic iPSCs with or without mutation and also provided a system that can be used to explore candidate therapeutic approaches.